In recent years, traffic in the internet continues increasing rapidly, and a demand for a high transmission capacity of a core network has been increased. To meet the demand in an existing 10 Gbps wavelength division multiplexed transmission system, the upgrade from a 10 Gbps optical signal to a 40 Gbps optical signal or a 100 Gbps optical signal has been requested.
In accordance with the request for high speed modulating signal, various approaches for an optical transmission technology have been investigated. Among them, the optical multilevel transmission technology as a new modulation system substituting for intensity modulation which is utilized in an existing wavelength division multiplexed transmission system gets a lot of attention. The multilevel optical transmission technology is enable to realize a large capacity of information transmission by utilizing a phase or an amplitude of optical signal and researches have been investigated on various modulation systems of a phase shift keying (PSK, Phase Shift Keying), a quadrature amplitude modulation (QAM, Quadrature Amplitude Modulation). Also, in optical receivers, there has been carried out an investigation which is combined not only with direct detection used in an existing wavelength division multiplexed transmission system, but also with various modulation systems of coherent detection, incoherent detection. Recently, as a modulation method of a 40 Gbps optical signal, RZ-DQPSK (Return-to-Zero Differential Quadrature Phase Shift Keying) using incoherent detection becomes mainstream, and optical transceivers and pieces of optical transmission equipment have been developed. Also in a modulation method of a 100 Gbps optical signal, PM-QPSK (Polarization-Multiplexed Quadrature Phase Shift Keying) using coherent detection gets a lot of attention, and research and development has been carried out on an optical transceivers.
Moreover, polarization multiplexing gets a lot, of attention. The polarization multiplexing is a method of transmitting two independent signals from each other by utilizing two polarization states having the same wavelength and orthogonal to each other. Due to this method, a modulation rate can be halved, and combined with the optical multilevel transmission technology described above, not only a high-speed modulating signal can be realized by low symbol rate, but a high transmission rate can be realized. Such polarization multiplexing is disclosed in, for example, JP-A-62-024731.